The 3rd Generation Partnership Project (3GPP) standardization body is currently working on the specification of the evolved 3G mobile system called Long Term Evolution (LTE) or E-UTRAN. This system will support inter-working with CDMA2000 networks (HRPD) and (1xRTT). The inter-working will be in the form of packet switched (PS) handovers for ongoing IP real-time services between LTE and HRPD as well as handover for ongoing Voice over IP calls from LTE to circuit switched (CS) 1xRTT networks (the latter mechanism is called Voice Call Continuity (VCC)).
The solutions above are described in 3GPP TS36.300 section 10.3.2, and which form part of the prior art. However recently it has also been considered important to introduce support for circuit switched (CS) fallback from LTE to legacy networks, such as Global System for Mobile communications (GSM) and 1xRTT. In short, by Circuit Switched Fallback mechanisms, the CS-domain services are realized by reuse of Circuit Switched infrastructure (radio and core network) and by Circuit Switched fallback is meant that the user equipment (UE) camps on a Packet Switched only system (e.g. E-UTRAN) but switch over to Circuit Switched system (e.g. GERAN/UTRAN) to establish originating Circuit Switched call.
For incoming terminating calls the user equipment is paged in the Packet Switched only system (e.g. E-UTRAN) but respond to the page in the Circuit Switched system. The Circuit Switched fallback mechanism differs from VCC in that the VCC mechanism is applied to on-going services while Circuit Switched fallback is intended to switch directly to the Circuit Switched domain, before setting up any Circuit Switched services.
The principle for the Circuit Switched fallback feature is that the mobile terminal performs registration, while in LTE, to the Circuit Switched domain. Once the terminal then wants to make a Circuit Switched voice call, or receives an incoming paging for a Circuit Switched voice call, it will leave the LTE domain and switch over to the Circuit Switched radio access network (e.g. 1xRTT, GSM) and initiate the call setup in the Circuit Switched network.
A drawback with the above described is that the access delay for the mobile to access the target Circuit Switched system will be quite long, which will have a negative impact on the service performance, i.e. the service interruption time will be long, in the order of seconds depending on the implementation. The reasons for this is that the user equipment need to perform cell search on the 1xRTT carriers, acquire synchronization, read information from broadcast channel etc. before it is able to respond to the incoming page or setup up the mobile originated calls.
It is of course possible to re-use concepts for Packet Switched to Circuit Switched handovers from LTE to 1xRTT in order to speed up the time it takes for the user equipment to access the 1xRTT system. The principle for LTE to 1xRTT handover is based on that resources and signaling are performed towards the 1xRTT system before the user equipment leaves LTE. The concept is also based on that they user equipment are performing measurements on 1xRTT cells prior to leaving LTE. These concepts not only rather complicated and have large impact on both the networks (3GPP and 3GPP2) and terminals, but they are also meant for ongoing services, not for redirection before initiating services.
Then there is also general concept defined for reducing service interruption time at cell change for ongoing Packet Switched based services (e.g. in GERAN/UTRAN), which are well known are used both for handover (when target cell is prepared) and for network assisted or controlled cell re-selection (when the target cell is not prepared but the access delay is still reduced since the user equipment does not need to read broadcast information), again these concepts are not designed for re-directing before initiating services and they are also not directly applicable to the Circuit Switched domain or to 1xRTT which uses other access methods and synch procedures.